Silica and carbon black have been commonly used as reinforcing agents and/or fillers in rubber and thermoplastic polymers. A number of techniques have been developed to incorporate such reinforcing agents and fillers into the polymer compositions, including both wet and dry blending processes. The incorporation of silica and carbon black as reinforcing agents and/or fillers is far more complex than might otherwise appear. One problem in wet blending of silica with water-based lattices of such polymers arises from the fact that the hydrophilic silica has a tendency to associate with the aqueous phase and not blend uniformly with the hydrophobic polymer.
Perhaps the most commonly employed practice, used commercially, is the technique of dry blending either silica, carbon black, or both into rubber or thermoplastic polymers in a high-shear mixer. This practice has many limitations. Notable among them include the tendency of the filler particles to agglomerate with each other, resulting in a non-uniform dispersion of the filler throughout the polymer constituting the continuous phase. Another problem commonly experienced in such high-shear mixing operations is the tendency of the polymers to degrade, or break down, during processing. This necessitates the use of higher molecular weight polymers, which sometimes require the incorporation of various types of processing aids to facilitate the mixing and dispersion of the filler particles into the polymer constituting the continuous phase. The cost associated with the use of such processing aids also increases the manufacturing cost of the polymeric compound or article. The use of processing aids has the further disadvantage in that such processing aids may have a negative effect on the cure or end-use properties of the polymeric compound or article. In addition, dry blending techniques add additional processing costs due to excessive equipment wear caused by the abrasive fillers.
To improve dispersion of the silica during dry mixing, it has been proposed that such compounding operations employ silica that has been pre-treated with an organosilane coupling agent having dual functionality. Representative of such coupling agents are those well-known compounds that include both an organic group, along with a readily hydrolyzable group, such as an alkoxy group as represented by a methoxy group or an ethoxy group likewise bonded directly to the silicon atom. In those systems, it is generally recognized that the alkoxy group hydrolyzes in the presence of moisture typically found on the surface of the silica to form the corresponding silanol that reacts with or condenses in the presence of the silica surface to bond the silicon atom to the silica surface. The polymer matrix may become chemically bonded by means of the coupling agent to the silica surface during cure or vulcanization of the polymer. Problems associated with the use of such silanes during compounding include the release of volatile organic compounds (VOC) due to the hydrolysis of the readily hydrolysable groups, unpleasant odors due to the presence of mercapto or sulfur containing organic groups, and premature curing and/or scorching. A need exists to provide a process that allows for the incorporation of silica into natural or synthetic polymers during the latex stage that overcomes the foregoing disadvantages.
There is a need to provide a process for the incorporation of silica reinforcing filler alone or with other fillers, particularly carbon blacks, into natural and synthetic polymers during the latex stage that is simple and inexpensive.
Another need is to provide a process for the incorporation of silica reinforcing filler alone or with other fillers, particularly carbon blacks, into natural and synthetic polymers such that the silica can be substantially—uniformly dispersed throughout the polymer matrix, and such that the silica can functionalized to react with the polymer matrix during processing or curing for end use compound or article.
A need also exists for a wet process for treating precipitated or fumed silica with a coupling agent whereby it becomes compatible with the natural or synthetic polymer phase of a polymer latex.
The present embodiments meet these needs.